Inkjet printers fall into two major technological families, a first family consisting of “drop-on-demand” printers and a second consisting of continuous jet printers.
“Drop-on-demand” printers are generally office printers, designed for printing text and graphic patterns in black or colour on sheet substrates.
“Drop-on-demand” printers directly and solely generate those ink drops effectively required for printing desired patterns. The print head of these printers comprises a plurality of ink ejection nozzles, usually aligned along a nozzle alignment axis and each addressing a single point of the print medium. When the ejection nozzles are in sufficient number, printing is obtained by simple movement of the print medium under the head, perpendicular to the nozzle alignment axis. If not in sufficient number, additional scanning of the medium relative to the print head is essential.
Continuous inkjet printers are generally used for industrial marking and coding applications.
The typical functioning of a continuous inkjet printer can be described as follows. Electrically conductive ink maintained under pressure escapes from a calibrated nozzle to form an inkjet. Under the action of a periodic stimulation device, the inkjet so formed breaks up at regular time intervals at a single point in space. This forced break-up of the inkjet is usually induced at a so-called jet break-off point by periodic vibrations of a piezoelectric crystal placed in the ink upstream from the nozzle. After break-off, the continuous jet turns into a succession of identical, regularly spaced ink drops. In the vicinity of the break-off point is a first group of electrodes called “charge electrodes” whose function is to transfer selectively and to each of the drops a predetermined quantity of electric charge. All the jet drops then pass through a second arrangement of electrodes called “deflection electrodes” forming an electric field which modifies the pathway of the charged drops.
In a first variant of so-called continuous inkjet deflection printers, the quantity of charge transferred to the ink drops is variable and each drop records a deflection proportional to the electric charge previously allocated to it. The point of the print medium reached by a drop depends upon this electric charge. Non-deflected drops are recovered by a gutter and recycled towards an ink circuit.
It is also known to persons skilled in the art that a specific device is required to ensure constant synchronization between jet break-off times and application of charge signals to the drops. It is to be noted that this technology, through its multiple levels of deflection, enables a single nozzle to print the entirety of a pattern in successive swathes, i.e. in lines of points of given width. Passing from one swathe to another is made via relative continuous movement of the substrate relative to the print head, perpendicular to said swathes. For applications requiring slightly wider printing width than the width of a single swathe, several single-nozzle print heads, typically 2 to 8, may be grouped within one same casing.
A second variant of continuous inkjet printers called binary continuous inkjet printers, sets itself apart from the previous variant chiefly through the fact that only one level of drop deflection is created. The printing of characters or patterns therefore requires the use of multi-nozzle print heads. The centre-to-centre distance of the nozzles coincides with the centre-to-centre distance of the impacts on the print medium. It is to be noted that in general the drops intended for printing are non-deflected drops. Binary continuous inkjet printers are intended for high speed printing applications such as addressing or personalizing of documents.
It is to be emphasized that the continuous inkjet technique requires ink pressurization, thereby allowing a print throw, i.e. the distance between the lower face of the print head and the print medium, possibly reaching 20 mm, i.e. ten to twenty times greater than the print distances of drop-on-demand printers.
The addressability of a continuous inkjet printer is the number of separate impacts per unit width of a printed swathee. For example, a single-nozzle continuous inkjet deflection printer equipped with a nozzle having a diameter of 50 micrometers, provides approximately 5 impacts per millimetre. The number of impacts in a swathee is in the order of 25. Under these conditions the maximum width of a swathe is typically 5 mm at usual printing distances.
For the same print quality, numerous applications require a slightly greater printing width, up to 10 mm under the conditions of the above-cited example.
One known solution to achieve such swathe widths consists of the binary continuous jet multi-nozzle print head briefly described above. These machines are rapid and enable swathe widths ranging up to 50 mm. For print quality similar to that of continuous inkjet deflection printers however a nozzle plate is required whose tolerances on the ink ejection orifices are very tight. Any difference in the diameter of the orifices translates as a different drop volume which in turn translates as a different size of drop impact. Tolerances for spacing and directionality of the orifices are also very tight since they determine the accuracy of the impact position.
It is also necessary to provide for a jet stimulation device enabling equal break-off distances for each jet. Said condition is difficult to implement in particular for jets from the end nozzles of the nozzle plate.
Design and manufacturing constraints in particular for nozzle plates and stimulation devices give rise to costs associated with binary continuous jet multi-nozzle heads, per print width unit, which largely exceed those associated with deflection continuous jet heads. Also if due heed is not given to these constraints, printing is of lesser quality.
Another known solution incorporates two nozzles in one same casing each nozzle ejecting an inkjet used according to the deflection continuous jet technique.
One first example of this solution is given in patent application WO 91/05663 (U.S. Pat. No. 5,457,484) in the name of the applicant. The head described in this application comprises two single-nozzle print heads mounted on one same support. Advantageously, there is only one ink recovery module with only one return channel for the two heads. The geometry of the heads, in particular the relative angle of the axes of the nozzles, and the deflection voltages of the drops derived from each of the two heads are adjusted to obtain juncture of the swathes printed by each of the two heads on the print medium, so that a single swathe is obtained having twice the width of the one obtained with only one head.
Juncture of two swathes is obtained by juxtaposing on the print medium the impact of the most deflected drop from one head with the impact of the least deflected drop from the other head, so that these two drops are positioned relative to one another as if they were two spatially consecutive drops from one same head. Precise juncture with no visible defect is difficult to achieve since the pathway and hence the point of impact of the most deflected drop is highly sensitive to aerodynamic and electrostatic disturbances set up in particular by the presence of other drops. In this embodiment, any change made to the volume of the formed drops will require review of the geometry of the printhead. One first reason derives from the fact that the pathway of a charged drop, especially the pathway of a highly charged drop such as the most deflected drop, varies in relation to the ratio between the electric charge and drop volume. It follows that the pathways of drops of different diameters are not identical. In particular, the impact points of the most deflected drops of different diameters will not be identical. A second reason derives from the fact that the maximum electric charge which can be applied to an ink drop depends upon its diameter. This means that one cannot simply compensate for a variation in drop volume by a variation in electric charge to obtain the same deflection. On this account, to achieve good juncture between the swathes formed by each of the heads, the geometry of the multi-nozzle head must be adapted to drop volume. Similarly, any difference in diameter of the orifices translates as different drop volumes which, for the same charge, has an influence on their deflection and hence on the accuracy of the impact on the substrate and consequently on juncture.
A second embodiment in which two nozzles are incorporated in one same casing, both nozzles each ejecting an inkjet treated according to the deflection continuous jet technique, is described in patent application WO 91/11327.
In the device described in this application the two heads may benefit from common structures such as the ink reservoir, the vibrator used to break up the jet into drops, and a central drop deflection electrode. The jets ejected from the two nozzles are parallel to one another. It is to be noted as is shown in FIG. 1 of this application that the plane defined by the axes of the jets is perpendicular to the plane containing the pathways of drops deflected by the deflection electrodes. As a result, if no special precautions are taken as explained hereafter, the two swathes do not lie within the extension of one another. The consecutive drops the closest to one another in each of the swathes which can be traced by one of the heads, i.e. the juncture drops of the two swathes, are the least deflected drops of each of the two swathes. Which means that this two-nozzle head does not have the same disadvantages as the first two-nozzle head for example. Since it uses common members it can be produced at less cost. A change in the diameter of the nozzles does not require adjustment of the direction of the nozzle axes to ensure juncture of the swathes.
This second embodiment has other disadvantages however. Firstly, as mentioned above, since the nozzle axes are parallel to one another and since the plane defined by the jet axes is perpendicular to the plane containing the drop pathways, it follows that the swathes traced by each of the jets when the medium is immobile are swathes parallel to one another. The distance between the straight lines carrying these two swathes is substantially equal to distance d separating the nozzle axes from each of the heads. During normal operation it was seen above that the heads and the medium have relative movement along a direction perpendicular to the swathes. Consequently, for the swathes traced by each of the heads to lie within the extension of one another, consideration must be given to distance d, to the speed of travel of the medium and to the flight time of the drops between their ejection and impact, in order to adjust delay between drop ejection times by each of the heads. This fact is not mentioned in the description of this second example other than in a passage on page 3 lines 16–18 where it is indicated that the electronic control circuits are within the reach of persons skilled in the art and will therefore not be described. Adjustment of the delay between the drops from each of the nozzles therefore assumes a specific circuit to manage this delay. Even if such circuit includes good servo-control of the delay relative to the speed of travel of the substrate, the joining of the swathes will continue to fluctuate on account of variations in travel speed and/or mechanical tension of the substrate and/or drop velocity over time leading to corresponding variations in drop position.
Other disadvantages are common to the heads of the first and second embodiments described above.